Most radio frequency (RF) transmitters use some type of power amplifier (PA) in order to amplify a signal prior to its transmission. This amplification is often associated with a non-linear distortive effect on the transmitted signal. For example, PAs may inflict non-linear distortion as amplitude-to-amplitude modulation (AM-AM) and amplitude-to-phase modulation (AM-PM) on a transmitted signal, which tend to be especially severe at high output levels. Consequently, non-linear distortion often limits practically achievable output power levels of a PA, thus reducing the reach of radio transmission and also degrading signal to interference and noise ratio (SINR) at a receiver of the transmitted signal.
In order to mitigate this type of distortion, digital pre-distortion (DPD) techniques are often deployed which compensate for the distortion from PAs. State-of-the-art DPD algorithms tend to be quite complex, requiring extensive processing power and memory resources, which drives production and operating costs of the communications equipment.
Large antenna array wireless systems, such as massive multiple-input multiple-output (MIMO) systems, exploit a large number of active antenna elements to transmit and receive wireless signals. By using large antenna arrays the capacity of a wireless communication system in terms of, e.g., information bits per second and Hertz can be increased compared to corresponding single-input single-output (SISO) wireless systems.
However, performing non-linear distortion mitigation such as DPD or similar becomes prohibitively complex in active antenna systems comprising a large number of nonlinear PAs. This is because electromagnetic mutual coupling effects between antenna elements, such as signal leakage between adjacent antenna elements, causes a large number of inter-related distortion components that are difficult to predict and compensate for by a DPD algorithm.
This type of distortion effect seen in large antenna arrays will henceforth be referred to as a mutual coupling-based non-linear distortion effect.
At least partly due to the above, there is a need for alternative distortion mitigation methods of reduced complexity for use in antenna array systems.